It is to be noted that the expression PCB can be used in industry to describe just the substrate of an SMA, particularly a substrate which has been etched or plated with a solder mask, but which has not yet had electronic components fixed thereto by soldering, or it can be used to describe a finished SMA which has electronic components fixed to the substrate. Reference herein to a PCB is intended to cover both of these uses of the expression PCB, although where appropriate, reference will also be made to the expression SMA, which will describe a completed assembly which includes a PCB as the substrate to which electronic components are fixed.
A PCB normally comprises a copper sheet which is bonded to one or both sides of a non-conductive substrate. The non-conductive substrate can be a resin for example or a nonporous polytetrafluoroethylene (PTFE). Parts of the copper sheet can then be removed, such as by etching, to create electrical paths on the surface of the substrate to connect electronic components. Alternatively, the copper paths can be added to the surface of the substrate by plating, but this is a less popular approach.
The electrical paths lead to points for component connection, so that two or more components can be connected electrically by connection to the board. The normal connection of the components to the copper paths of the PCB is by solder. Where copper sheet is bonded to both sides of a non-conductive substrate, electrical connection can be made between the copper sheets on either side of the substrate by holes drilled through the substrate (known as “vias”) which are plated with a conductive metal, i.e. copper. Thus, there is an electrical bridging between the copper sheets so that components can be connected to either side of the PCB and still be in electrical connection.
Some PCBs include more than two layers of copper with one or more layers embedded in the substrate. In these PCBs adjacent layers can be connected by vias that do not necessarily extend fully through the substrate.
Exposed copper paths can be covered with a suitable coating to prevent damage to the paths by corrosion. The coating can be a solder mask, or a nickel or gold layer for example. The points for component connection can also be covered for corrosion protection, although the coverage can be different so as not to prevent connection of a component to the PCB by solder. In some PCBs, the connection points are plated with tin which prevents the points from corroding prior to component connection, but which melts away easily during the soldering process to expose the connection surface.
SMAs including a PCB substrate can be manufactured almost entirely automatically. As a result, SMAs are relatively inexpensive and suitable for high volume manufacture. To ensure reliability of operation of an SMA, part of the manufacturing process involves inspection of each solder connection between an electronic component and the PCB substrate. Advantageously, the inspection process can be by automatic optical inspection (AOI).
AOI is very suitable for inspecting solder connections which are visible on the surface of the PCB once the connection has been made. The AOI process is also suitable for inspecting connections on both sides of a PCB as the PCB can be easily turned from one side to the other. However, some electronic components are connected to the PCB via connections that are not visible once they are made. For example, the base of a component known as a “semiconductor package” or an “integrated circuit” can include a component known in the industry as an “exposed pad” or otherwise known as a “heat slug” (hereinafter an “exposed pad”) which connects to the PCB substrate by a solder connection that is not visible once the connection is made. Exposed pads can be provided for example to conduct heat away from the semiconductor package or integrated circuit and to improve the electrical connection of the package or circuit to the PCB substrate. Thus, exposed pads can increase the removal of waste heat and the delivery of electric current to the package or circuit so that the capacity of the component and thus the SMA can be increased.
An exposed pad is normally connected to the PCB substrate by a solder connection. That solder connection once made, is hidden or concealed between the semiconductor package or integrated circuit and the PCB substrate. Connections of this kind are generally made in addition to the normal visible connections made for these components and known to persons skilled in the art.
Hidden joints of the kind discussed above still require inspection to ensure a proper and secure connection has been made between the component and the conducting surface of the substrate. To date, X-ray inspection for hidden or concealed connections has been made. However this is costly in terms of the initial investment in the X-ray equipment, and inspection by X-ray introduces an extra step in the inspection process and thus increases the cycle time for SMA manufacture.